1. Field
The presently disclosed subject matter relates to a light guide plate and a side-edge type surface-emission optical apparatus including the light guide plate used in a liquid crystal display (LCD) unit.
2. Description of the Related Art
Side-edge type surface-emission optical apparatuses used for LCD units are advantageous in terms of their thin and light structures.
FIG. 15A is a plan view illustrating a prior art side-edge type surface-emission optical apparatus, and FIGS. 15B and 15C are cross sectional views of the side-edge type surface-emission optical apparatus of FIG. 15A taken along the lines B-B and C-C, respectively, in FIG. 15A (see: FIGS. 1(a) and 9 of JP2009-81094A & US2009/0086509A1).
In FIGS. 15A, 15B and 15C, the side-edge type surface-emission optical apparatus is constructed by a light guide plate 1 made of transparent material such as acryl resin or polycarbonate resin with a light incident surface Sin, a light distributing control surface Scont perpendicular to the light incident surface Sin and a light emitting surface Sout opposing the light distributing control surface Scont. A light source 2 formed by a plurality of light emitting diodes (LEDs) is disposed on the light incident surface Sin. Also, a prism sheet 3, which is illustrated only in FIG. 15C, is disposed on the light emitting surface Sout of the light guide plate 1, and a Liquid crystal display (LCD) panel (not shown) is disposed outside of the prism sheet 3. The prism sheet 3 is a single-face-deformed triangular prism sheet; however, the prism sheet 3 may have a cross section such as a curved cross section other than a triangular cross section. Further, a flexible reflective sheet 4, which is illustrated only in FIGS. 15B and 15C, is disposed on the light distributing control surface Scont. The flexible reflective sheet 4 is operated to return light leaked from the light distributing control surface Scont thereto, thus enhancing the luminous intensity. In FIGS. 15B and 15C, note that the flexible reflective sheet 4 is apart from the light guide plate 1; however, the flexible reflective sheet 4 is actually in close proximity to the light distributing control surface Scont of the light guide plate 1.
As illustrated in FIG. 15A, a plurality of flat mirror-finish portions 11 are provided on the light distributing control surface Scont of the light guide plate 1 and extend from the light incident surface Sin. The flat mirror-finish portions 11 serve as means for spreading light to the inner part of the light guide plate 1. In this case, the farther from the light incident surface Sin a location of the flat mirror-finish portions 11, the smaller the width of the flat mirror-finish portions 11 at that location. A plurality of triangular prism sequences 12 are provided on areas of the light distributing control surface Scont of the light guide plate 1 where the flat mirror-finish portions 11 are not provided. The triangular prism sequences 12 are protruded with respect to the flat mirror-finish portions 11. Each of the triangular prism sequences 12 includes a plurality of equidistantly-arranged triangular prisms for bending the path of light. In this case, the farther from the light incident surface Sin a location of the triangular prism sequences 12, the larger the width of the triangular prism sequences 12 at that location. Thus, much more light is totally reflected by the triangular prism sequences 12, to realize a uniform surface emission.
In FIG. 16, which is a partly-enlarged cross sectional view of the light guide plate 1 of FIGS. 15A, 15B and 15C, each prism 12P of the triangular prism sequences 12 per pitch P0 has an asymmetrical structure formed by a straight-type rising sloped surface 12a whose slope angle is defined by α0, and a straight-type falling sloped surface 12b whose slope angle is defined by α1 (≠α0). Particularly, the falling sloped surface 12b carries out a light distributing control to bend the path of light. In order to enhance the luminous intensity of a surface-emission along the normal direction to the prism sheet 3, the slope angle α1 of the falling sloped surface 12b with respect to the flat mirror-finish portions 11 is preferably as small as possible, for example.4°≦α1≦5°As a result, spatial light distribution characteristics of the light guide plate 1 with a narrow full-width at half maximum as shown in FIG. 17 can be realized. In FIG. 17, note that the full-width at half maximum is 25° with a range from 55° to 80°. Also, the slope angle α0 of the rising sloped surface 12a with respect to the flat mirror-finish portions 11 is15°≦α0≦90°
Thus, light distribution characteristics of the prism sheet 3 as shown in FIG. 18 with a high emission along the normal direction to the prism sheet 3 can be realized.
In the side-edge type surface-emission optical apparatus of FIGS. 15A, 15B and 15C, in order to further enhance the luminous intensity along the normal direction to the prism sheet 3, the slope angle α1 of the falling sloped surface 12b with respect to the flat mirror-finish portions 11 is preferably smaller than 4″. As a result, the falling sloped surface 12b on the side of the light source 2 is approximately in parallel with the flat mirror-finish portions 11. Therefore, as illustrated in FIG. 19, the flexible reflective sheet 4 is in broad contact with the falling sloped surface 12b so that a region R with no air gap may be generated between the falling sloped surface 12b and the flexible reflective sheet 4. In the region R, the flexible reflective sheet 4 partly spreads light reflected at the falling sloped surface 12b, so that a so-called wet-out phenomenon inviting irregular patterns as illustrated in FIG. 20 may occur in the light emitting surface Sout of the light guide plate 1, which would degrade the light distribution characteristics of the prism sheet 3 of FIG. 18.